This invention relates to oxidation catalysts. More particularly this invention comprises a new composition of catalyst, method of making the catalyst and processes for its use.
The prior art is replete with teachings of the use of all metal catalyst elements, as well as teachings of the preparation of such types of catalysts, in connection with the treating of noxious gases and vapors. The advantage of a catalytic incineration of volatile combustion materials from industrial processing operations resides in the lowered heat requirements and the large resulting annual savings in fuel costs such that a higher installation cost can be quickly offset. For example, a thermal incineration operation for a gaseous stream may require a temperature in the range of 1100.degree. to 1500.degree. F.; while, on the other hand, the catalytic operation may be carried out at an inlet temperature range of from 600.degree. to 700.degree. F. The fuel savings are obvious.
The all metal catalyst units may be made in various forms, as for example, as one or more layers of coated screen of different gauges and meshes, short lengths of coated wire or ribbon in coiled form, coated mats of crinkled wire or of crimped strands of ribbon, etc. A popular form of unit has comprised a mat of crinkled or crimped thin and narrow ribbon placed between retainer screens of wire mesh. The elements have been made in various thicknesses, ranging from about 1 to 3 inches in thickness. Also, it has been of advantage to place the wire or mat portions of the elements into stainless steel frames such that they can be readily placed and held in an exhaust gas stream flow path.
The base materials for the noble metal coatings used in the heretofore known types of elements have been of special high nickel content alloys of various types, such as Chromel, Nikrothal, and Nickrome, (which are the trade names for, respectively, an alloy of nickel and chromium and an alloy of nickel, chromium and iron). These alloys provided the desired requirements of having high thermal and electrical resistance in comparison to the usual or more widely used stainless steels that had lower nickel content.
A specific example of such an earlier teaching is as set forth in U.S. Pat. No. 2,658,742 to Suter et al. Suter et al discloses the use of a noble metal supported on a metallic alloy for use in the catalytic incineration of combustible vapors. The noble metal may, in accordance with Suter et al, be deposited on the base alloy by electroplating which was stated to be the preferred method of deposition because it prevents a replacement reaction whereby the noble metal replaces a base metal which enters and contaminates the plating solution.
U.S. Pat. No. 3,867,313 to Brewer also discloses the use of an incineration catalyst made by electroplating a noble metal on an alloy base. Brewer, however, teaches that the problem of dissolved cation impurities from the base metal still arises even though the method of plating is not based on the replacement reaction. Brewer solves that problem by circulating the plating solution through a cation exchange resin. Brewer also teaches the conditioning of the catalyst by contacting it at high temperature with a mixture of hydrocarbon combustion gases and hydrocarbon vapors.
U.S. Pat. No. 3,208,131 to Ruff et al discloses a catalyst unit in suitable form for use as an automotive exhaust gas catalytic converter. The unit comprises spirally wound alternating layers of crimped and flat alloy ribbon to form a honeycomb-like passage for the gases. The ribbon is coated with a thin layer of noble metal to achieve the catalytic effect.
U.S. Pat. No. 4,162,235 to Acres et al discloses the possibility of a catalyst composition containing a noble metal and a rare earth metal carried by a substrate which may be a metal, in which case a perovskite must also be present, or a composition without the perovskite, in which case a support comprising a refractory oxide is employed in addition to the substrate. The only specific means suggested for adding the rare earths and noble metals to the support or substrate is by impregnation or "dipping". No mention is made of electroplating.
Other references in the art disclosing various rare earth-noble metal combinations for use as oxidation catalysts are U.S. Pat. No. 3,993,572 to Hindin et al, U.S. Pat. No. 4,140,655 to Chabot et al, U.S. Pat. No. 4,118,199 to Volker et al and U.S. Pat. No. 3,997,651 to Bocciarelli et al. None of these references suggests electroplating as a means of introducing active metals to support materials, nor, of course, the specific details of an electroplating procedure which, as will be hereinafter discussed, are preferred by the present invention.